The invention relates to a safety circuit for an elevator installation consisting of a chain of switches connected in series to monitor the equipment serving the safety of the elevator operation, and of a source of electric voltage to supply the series chain, there being connected to the end of the series chain at least one switching device which generates signals for an elevator control depending on the switching status of the switches.
A safety circuit for an elevator installation consists of a chain of door contacts connected in series, a contact being provided for the purpose of, for example, monitoring the position of a hoistway door. Further contacts or switches for the purpose of monitoring, for example, the position of the car door, the position of the brake, or other equipment serving the safety of the elevator operation, can be connected into the safety circuit. The safety circuit is usually supplied with impulses of direct voltage from either an AC or DC source of voltage, there being connected to the end of the safety circuit at least one safety relay. If all contacts are closed, the safety relay is activated. The elevator control monitors the status of the safety relay and if the safety relay is activated, the elevator control releases, for example, a pending travel command.
A disadvantage of this type of electrical supply to the safety circuit is that the output voltage of the voltage source is not regulated and is subject to voltage fluctuations which in turn makes relays with a wide voltage range necessary. Furthermore, the voltage has a value greater than a safe low voltage, and to prevent electrical accidents must be protected with a fault-current safety switch.
It is here that the invention sets out to provide a remedy. The present invention provides a solution to avoiding the disadvantages of the known device and creating a safety circuit that operates safely irrespective of the travel height of the elevator.
The advantages derived from the invention are essentially that the voltage across the safety relay is held constant. The voltage across the safety relay therefore no longer depends on the length of the cabling of the safety contacts, which is of particular significance on elevator installations with very high travel. The cabling of the door contacts extends over the full height of the elevator hoistway and, if there is no regulation, has a direct influence on the voltage across the safety relay. If the voltage is regulated, power supply voltage fluctuations, or changing contact resistances on the contacts, or other loads in the safety circuit which influence the voltage, have no effect on the safety relay. If the voltage across the safety relay is regulated, a commercially available standard relay can be used as the safety relay without detriment to the reliable operation of the safety circuit. Moreover, the safety circuit can be operated with physiologically safe low voltage. In particular, measures for the protection of persons are not necessary. With regulated voltage across the safety relay, a safety circuit can be made with high operational safety and low costs.
When the safety circuit is open, a limiter acting through a network of the regulating circuit limits the supply voltage to a specific value. Moreover, the safety circuit operates with a low voltage which is not dangerous to persons.